Some multi-band or other tactical radios operate in the high frequency (HP), very high frequency (VHF) (for satellite communications), and ultra high frequency (UHF) bands. The frequency range of these multi-band tactical radios can operate over about 2 through about 512 MHz. Next generation radios will probably cover about 2.0 to about 2,000 MHz (or higher) to accommodate high data rate, higher bandwidth waveforms and less crowded frequency bands. In the HF frequency band the transmit mode is governed by standards such as MIL-STD-188-141B, while data modulation/demodulation is governed by standards such as MIL-STD-188-110B, the disclosures which are incorporated by reference in their entirety.
UHF standards, on the other hand, provide different challenges over the 225 to about 512 MHz frequency range, including short-haul line-of-sight (LOS) communication and satellite communications (SATCOM) and cable. This type of propagation can be obtained through different weather conditions, foliage and other obstacles making UHF SATCOM an indispensable communications medium for many agencies. Different directional antennas can be used to improve antenna gain and improve data rates on the transmit and receive links. This type of communication is typically governed in one example by MIL-STD-188-181B, the disclosure which is incorporated by reference in its entirety. This standard provides a family of constant and non-constant amplitude waveforms for use over satellite links.
The joint tactical radio system (JTRS) is one example of a system that implements some of these standards and has different designs that use oscillators, mixers, switchers, splitters, combiners and power amplifier devices to cover different frequency ranges. The modulation schemes used for these types of systems can occupy a fixed bandwidth channel at a fixed frequency spectrum. These systems usually utilize a memoryless modulation, such as phase shift keying (PSK), amplitude shift keying (ASK), frequency shift keying (FSK), quadrature amplitude modulation (QAM), or modulations with memory such as continuous phase modulation (CPM) and may sometimes combine them with a convolutional or other type of forward error correction (FEC) code. Minimum shift keying (MSK) and Gaussian minimum shift keying (GSMK) (together referred to as MSK or GMSK) are a form of continuous phase modulation used in the Global System for Mobile communications (GSM) and can be used with such systems. The circuits used for implementing the MSK waveform could include a continuous phase frequency shift keying (FSK) modulator.
Briefly, an MSK modulated signal can be considered as two combined orthogonal signals or channels that are 90 degrees out of phase with each other. Typically, each phase reversal is keyed to represent alternate bits of a binary signal that is to be transmitted. Each keyed pulse period could have a duration of a two bit period that is staggered by a one bit period, and when binary data is used to modulate each channel, the channels can be amplitude modulated with a positive or negative one-half wave sinusoid and combined by addition. Because the sine shaped envelopes of the two channels are 90 degrees out of phase with each other, the sum of the two channels results in a signal with a constant envelope, which could be amplified by non-linear class-C amplifiers and transmitted. A Gaussian filter having a Gaussian impulse response can be used for prefiltering symbols prior to any continuous phase modulation, thus forming a Gaussian minimum shift keying.
Spread Spectrum (SS) modulation spreads a waveform in frequency and typically provides robust data performance. SS modulation can use underlying orthogonal spreading sequences (i.e., Walsh Hadamard sequences) or pseudo-orthogonal spreading sequences (i.e., sequences obtained from maximum length shift-register sequences, shortened Walsh symbols, overloaded Walsh symbols, gold codes, or others).
Typically, two signals x (t) and y (t) are orthogonal when the average of their product x (t) y (t) equals zero. x (t) and y (t) can be random or Pseudo-random Noise (PN) signals and can be near-orthogonal (i.e., pseudo-orthogonal) with their products being zero in the mean, but sometimes not identically zero for all signal pairs. Any signals transmitted in these spread spectrum system are typically received and decoded and correlated in matched filters and/or signal processors that correlate a correlation function between a reference signal s(t) and the received signal r(t).
One common example of SS orthogonal data modulation is M-ary Walsh modulation. For example, IS-95 uses a 64-ary orthogonal Walsh modulation combined with PSK to send 6 bits of information. By definition, Walsh symbols are a group of M vectors that contain M binary elements in which every Walsh symbol of a given length is orthogonal to all other Walsh symbols of that length and all inverses of the other Walsh symbols of that length. For example, some systems use Walsh symbols having 64 chips to identify the logic channels. On both the forward and reverse channels the Walsh symbols have orthogonality. Walsh symbols can be produced using a simple iterative technique utilizing a base Walsh matrix.
In some current high performance radio network systems, it has been found that better performance can be achieved when robust, burst waveforms are used for the control, status and lower data rate data messages within the communications network. Many radio network systems, for example, such as manufactured by the assignee of the present invention, Harris Corporation of Melbourne, Fla., have used orthogonal Walsh modulation schemes to achieve the necessary level of robustness. Most radio frequency (RF) power amplifiers are peak power limited, however. For example, average power transmitted for a filtered SS phase shift keying (PSK) waveform can be several decibels (dB) less than the peak power capability of an RF amplifier because of the back-off required to accommodate the waveform's peak-to-average ratio. A constant amplitude waveform advantageously addresses this issue, but it is also desirable to maintain robustness and provide an adequate level of capacity (bps/Hz).
As known to many skilled in the art, constant amplitude or envelope waveforms are typically required for class C or class E amplifiers used in small handheld or personal radios. These types of power amplifiers are more efficient than the linear class A and class B amplifiers used in other radios. Class C and Class E power amplifiers are generally more efficient than linear RF power amplifiers (i.e., class A, AB, or B). The design challenges when using these types of waveforms is to maintain the constant envelope (CE) while also providing an adequate level of capacity (bps/Hz).
Constant amplitude (or envelope) waveforms are becoming increasingly important for handheld communications devices or radio devices. Gaussian Minimum Shift Keying (GMSK) and similar waveform variations used by those skilled in the art often are the basis for many radio waveforms used in such devices, but their communications are limited typically to one (1) bps/Hz. Performance issues, however, often cancel any gains resulting from the use of constant envelope waveforms, especially in smaller battery operated radios often used in a military or some commercial environments.